1. Field of the Invention
The invention relates to a blood pressure and pulse oximeter monitor apparatus that decreases the chance that a spurious loss of pulse signal from the pulse oximeter when the blood pressure cuff is being inflated and deflated will trigger an audible alarm.
2. Description of Related Art
When a pulse oximeter sensor is placed on the same limb as a blood pressure cuff, inflation of the cuff above systolic pressure will eliminate the plethysmographic pulse signal in the finger. See, for example, Pulse Oximetry, Payne and Severinghaus; Springer-Verlag 1986. A pulse oximeter measures the oxygen saturation of arterial blood by comparing the ratio of red light transmission through a pulsatile vascular tissue to infrared light transmission through the same vascular tissue. The absence of either pulsatile signal will cause the pulse oximeter to alarm. The loss of a pulsatile signal can indicate that the sensor is improperly attached, that the patient has no pulse, or some intervention has temporarily eliminated pulsatile flow. Since the loss of the pulsatile signal due to the inflation of the cuff is an artifact and not an indication of a fundamental change in the physiological status of the patient, it is desirable for the oximeter not to alarm in such a case.
Attempts have been made in the prior art to deal with the problem of setting off the pulse oximeter alarm during pressure cuff inflation/deflation. One technique described is in U.S. Pat. No. 4,776,339 entitled INTERLOCK FOR OXYGEN SATURATION MONITOR AESTHESIA APPARATUS discloses a system, including an activatable control means, which is operative to respond to the inflation of the cuff to provide a signal for disabling the oxygen saturation alarm for the time during which the cuff is inflated. Such devices suffer from two major problems. First, they disable more alarms than are necessary. For example, changes in oxygen saturation during the inflation/deflation of the cuff may be physiological and not caused by an artifact, (i.e. the cuff). Second, the normal cuff inflation/deflation cycle time is between 30 and 120 seconds. The pulses detected by the oximeter sensor, however, only fall below a minimal level for a period of time significantly less than that, e.g., 10 seconds. Therefore, it is only necessary to interlock the alarm for a period of roughly 20 seconds rather than interlock against the alarm for a complete cuff inflation/deflation cycle of 30-120 seconds. During the prior art interlock cycle, important alarms may have provided useful alarm information that the system was not capturing or reacting to.